1. Field of the Invention
This invention relates to plasma generation, and in particular, relates to a method of plasma generation utilizing microwave energy.
2. Description of the Related Art
Plasma generation, the generation of glow-discharge (a gas consisting of a stream of high velocity ions, electrons and neutral particles), is used to treat areas to be soldered in plasma-assisted dry soldering (PADS). See for example, U.S. Pat. No. 4,921,157 on a fluxless soldering process. Generation of plasma with microwaves is now done utilizing a low-pressure, rarified gas and generating plasma only in a small area. For example, a quartz insulator having dimensions of about 3-4 inches on a side is used to generate plasma with a small aperture of about 2 inches. In one type of plasma generation, using SF.sub.6 as a source gas, atomic fluorine F, is readily formed. It is a free radical and therefore has a short lifetime. Because there is only a small area of treatment which may be remote from some of the areas to be treated with the plasma, some areas may not be treated because the F does not survive until it reaches that treatment area. Thus, in the prior method, only a small portion of the active species, for example, F, actually gets to the treatment site. By the time the F reaches the treatment site, it has generally recombined with other radicals which have diffused out of the plasma region, often with about a 100-fold decrease in effectiveness at the outer regions of, for example, a 12-cm treatment area.
Plasma may also be generated with electron cyclotron resonance systems (ECR), inductively coupled (or capacitively coupled) radio frequency (RF) plasma, DC-coupled plasma systems and helicon waves (waves of plasma formed in a magnetic field which interact with the field in such a way as to heat the plasma). In addition, the AFTERFLOW.TM. hollow quartz tube (Machine Technologies, Inc., Parsippany, N.J.) within a microwave guide assembly may be used to generate plasma in a confined area.
It is therefore an object of this invention to provide a method and apparatus which allow plasma generation over an expanded area, for use, for example, in treatment with short-lived species, such as in plasma-assisted dry soldering.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.